This type of plastics coated tube and a method for its manufacture are discussed in U.S. Pat. No. 4,370,186. That document describes a method of manufacture in which a strip of aluminum is folded into a tubular form, seamed, for example by welding, and then passed over a mandrel. Plastics material is then extruded from the mandrel onto the inner surface of the aluminum tube and the tube then passes through a bush where a layer of plastics material is extruded onto the outer surface of the tube. The plastics coatings may then be cooled, for example in a water bath. In addition, European Patent 0024220 and British Patent Application 8625432 describe various aspects of the manufacture of such a plastics coated metal tube.
One preferred feature of the manufacturing procedure is to inject air or other gas under pressure into the tube as it is formed, from a gas line extending through the core of the mandrel. Provided that the leading end of the tube is sealed, a permanent pressure is then created within the tube and this assists not only in laying the extruded plastics coating onto the inner surface of the metal tube, but also in detecting any faults that may have occurred in the welding of the aluminum strip to form the tube. Thus, any faults in the weld will cause air to escape from the tube since the plastics coating on the internal and external surfaces of the coated tube is still relatively soft for a time immediately after being extruded onto the metal tube and the gas pressure within the tube will blow through the soft plastics material at any point where there is a fault in the weld. This escape of gas can be detected by a sensing mechanism which may be provided to sense a drop in pressure within the tube at any point downstream of the welding station, or by the appearance of bubbles of gas escaping from the tube as it passes through the cooling bath.
The leading end of the tube can be sealed quite satisfactorily by pinching and flattening the end while the internal plastics coating is still soft, or by pinching and flattening the end and bending the flattened end back on itself if the internal plastics coating has already cooled.
When the cooled tube is passed to a take up roll the tube needs to be severed only infrequently, generally when the take up roll is full. Accordingly, the operation of pinching and flattening the leading end of the tube is equally infrequent when a take up roll is used. However, when the tube is to be produced in relatively short lengths, it is necessary to effect very frequent severing of the tube and consequential sealing of the new leading end thereby created. This in turn, can lead to frequent release of the gas pressure within the tube and the severe risk that any welding faults which may occur before the new leading end of the tube is sealed and which can be detected with reliability only when the internal and external plastics coatings are still soft, will go undetected.